1. Field of the Invention
The present invention relates to a polyvinyl alcohol film composition suitable for use as an optical film, packaging film, agricultural film, laundry bags, cosmetic film and particularly, polarizing film.
2. Description of the Related Art
Polyvinyl alcohol (PVA) is a stable, non-toxic water-soluble polymer, which has a good adhesion, elongation, solvent resistance, wear resistance, oxygen barrier properties, and surface activity. Due to the desirable characteristics, PVA is used in various industries, such as the electronics, opto-electronics, textile, latex, plywood, and so on. In addition, PVA has good film-forming properties. PVA films have better physical properties, such as tensile strength, tear strength, and wear resistance, than films made of other water-soluble polymers. Typically, PVA films are prepared by solvent casting, wherein a PVA solution is coated by a drum type or belt type coater, and dried in a thermal process. PVA films may be subjected to water-swelling, monoaxial stretching, and dyeing to form polarizing films. A polarizing film is typically combined with triacetyl cellulose (TAC) films to provide a polarizing plate, one of the key components in display devices.
A polarizing film having a high degree of polarization can be obtained by using a high stretch ratio during monoaxial stretching. To increase elongation of PVA films, a plasticizer is usually required. Plasticizers commonly used for PVA films include glycols, polyols, and amines, and the most common plasticizer is glycerine. A problem with PVA films is the migration or exudation of the plasticizer to the film surface, resulting in a sticky surface. In addition, because the plasticizer is substantially water-soluble, a considerable amount of the plasticizer is lost in the aqueous processing tanks during fabrication of polarizing films. Such tanks include swelling tanks, iodine-dyeing tanks, stretching tanks, and fixation tanks. The loss of the plasticizer can cause undesirable stiffening of PVA films, which ultimately leads to degradation of elongation, physical properties, and optical properties.
References may be made to Japanese Patent Publication Nos. 10-003007, 2006-307058, and 2007-154000, for further description of conventional PVA film compositions and their fabrication.
Accordingly, it is highly desirable to provide a PVA film composition having a low water leachability, wherein the loss of the plasticizer in processing tanks during fabrication of polarizing films can be reduced.